1. Field of the Invention
The present invention relates to a development device for forming a toner image on a predetermined image carrier, and an image forming apparatus having the same.
2. Description of the Related Art
An image forming apparatus such as a copying machine, printer, facsimile, a multi-function machine thereof and the like which employs the electrophotographic system includes a development device for supplying a toner to an image carrier (for example, a photoreceptor drum or a transfer belt) and forming a toner image on the image carrier.
The development device includes, as its basic constituent elements, a developer storage part for storing the developer, a development roller for receiving the developer from the developer storage part and forming a toner image on the image carrier by supplying the developer to the image carrier, and a regulating blade which is placed opposite to the development roller so as to form a regulatory gap with the development roller and which regulates the layer thickness of the developer on the development roller.
With this kind of development device, in order to form a favorable toner image on the image carrier, it is necessary to cause the developer layer to be thinned and uniform prior to the development roller supplying the developer to the image carrier.
With the development device of a first relevant technology, a magnet member is disposed on the upstream surface of the regulating blade when viewed from the rotating direction of the development roller. A magnetic field is generated between the regulating blade made from a magnetic material and a regulating pole disposed on the development roller, and the magnet member causes the magnetic flux density of the magnetic field to increase, and the magnetic line of the magnetic field is concentrated at the upstream portion in the regulating blade. The developer layer (magnetic brush layer) is thereby regulated and thinned and becomes uniform.
Moreover, with the development device of a second relevant technology, the development roller has a built-in magnet roll for pumping the developer onto its outer peripheral surface, and the regulating blade is formed from a magnetic material. Since the magnetic field of the magnetic pole opposite to the regulating blade in the magnet roll is concentrated on the regulating blade, the regulating blade is able to regulate the developer layer (so-called magnetic brush layer) on the outer peripheral surface to become uniform in the regulatory gap, which is a range that is affected by the magnetic field.
Nevertheless, with the development device of the first relevant technology, the regulating force of the regulating pole, the regulating blade and the magnet member for regulating the developer in the regulatory gap; that is, the restraint of the regulating pole, the regulating blade and the magnet member for restraining the developer in the regulatory gap is too strong. Thus, the stress applied to the developer upon regulating the magnetic brush layer increases, and the developer tends to deteriorate. If the developer deteriorates, it becomes difficult to form a favorable toner image on the image carrier, and consequently becomes difficult to form a favorable image on a sheet.
Moreover, with the development device of the second relevant technology, since the amount of developer that is transported to the regulating blade is considerably more than the amount of developer that passes through the regulatory gap, the developer tends to accumulate in a range around the regulating blade that is not affected by the magnetic field. The accumulated developers mutually collide or collide with the wall part defining the developer storage part, and will thereby deteriorate. In addition, an accumulated developer tends to fall into a compressed state, or a so-called packed state, and will thereby deteriorate. If the developer deteriorates as described above, it becomes difficult to form a favorable toner image on the image carrier.